The present invention relates to communication systems and methods, and more specifically to using differential input buffer circuitry to provide termination calibration circuitry.
Data rates at circuit input and output pins have increased dramatically the last several years, and further increases are sure to follow. These high data rates place strict requirements on signal integrity. Transmitted and received signals need to be substantially free from glitches and other undesirable artifacts to maintain the highest possible data rates.
Reflections due to impedance mismatches are one source of these artifacts. Reflections are caused when a signal is received from a transmission line by a receiver having an impedance that does not match the impedance of the transmission line. This mismatch prevents some of the energy of the signal from being absorbed by the receiver. This unabsorbed energy is reflected back to the signal source. To prevent these reflections, a receiver termination impedance is connected from a receiver input to a ground voltage, where the termination impedance matches the impedance of the transmission line. The receiver termination impedance is in parallel with the receiver, so it is referred to as a parallel termination impedance. Similarly, reflections can be caused by mismatches between a transmitter and its transmission line, so a source termination impedance is connected between a transmitter output and the transmission line. A source impedance is in series with the transmitter, and is referred to as a series impedance.
When a circuit is used to both transmit and receive data, an input/output stage including a plurality of transistors can be configured to provide a series termination when transmitting and a parallel termination when receiving, thereby providing On-Chip Termination (OCT). The impedance provided by the input/output stage can be adjusted by enabling or disabling individual transistors within the plurality of transistors.
The impedance provided by the input/output stage needs to be calibrated in order to match the impedance of the transmission line and in order to account for variations due to supply voltage, temperature, and other environmental factors. During calibration, the impedance can be adjusted to be substantially equal to a reference resistance.
A system for calibrating termination impedance can be added to an existing system having an input/output stage. However, prior calibration circuits require additional external components to be added to the existing system in order to implement termination calibration functionality. More specifically, prior systems require external analog comparators to be added to the existing system in order to monitor the difference between an external resistance and an impedance of the existing system's input/output stage. Prior systems also require the use of additional digital logic to actively adjust the impedance of the input/output stage according to the comparators' outputs.